Three dimensional reflex lens systems

ABSTRACT

Optical systems for use with attachments for a conventional reflex motion picture camera to permit stereographic cinematography are disclosed. The optical systems include lens systems having four powered elements having negative, positive, negative, and positive focal lengths respectively. The lens systems are adapted to be used with the Bernier stereographic cinematography attachment system as modified for reflex motion cameras. The lens systems include a normal lens system, a wide angle lens system, and a telephoto lens system. The wide angle lens system further includes plane parallel refractive elements to increase its overall length without changing its back focal length, and the telephoto lens system is adapted to be used with internal displacement prisms which bring the images produced by the telephoto lens system into the camera flange opening.

This is a division of application Ser. No. 814,549, filed Dec. 30, 1985, now U.S. Pat. No. 4,688,906.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to photographic lens systems, and more particularly, to lens systems adapted for use in three-dimensional cinematography.

2. Description of the Prior Art

Three dimensional motion picture photography is old in the art. Many devices are known for photographing images from two positions separated by an interocular distance to place right- and left-hand adjacent images onto a frame of a film strip, side-by-side or one above the other. Bernier, U.S. Pat. No. 3,531,191, the disclosure of which is incorporated by reference herein, discloses an attachment for a conventional motion picture camera to permit the filming of stereoscopic images on motion picture film. The images are placed one above the other, rather than side-by-side.

The Bernier attachment comprises a pair of wide angled right and left eye displacement prisms which receive right and left eye images side by side and then transpose these images into vertical relationship on the film. The right and left eye images together occupy the same area on the film normally occupied by a single photographic frame. Immediately behind the prisms and part of the same attachment are two lens systems which gather the right and left eye images and focus them on the film plane. The two lens systems should be of the same equivalent focal length and in focus together so that the right and left eye images are on the same scale. The prisms and the lenses must fit tight packaging constraints in order to be mounted on a conventional motion picture camera. The prisms are folded to clear the flange requirements of the particular camera being used and the lens systems are actually located in the interior of the camera body. The prisms and lens systems of the three dimensional photography attachment must fit in the space normally occupied by a single lens system when the camera is not being used for three-dimensional cinematography.

The Bernier invention provided apparatus for attachment to a conventional motion picture camera such as the Mitchell BNC camera. This camera is a viewfinder camera and is being superseded by reflex motion picture cameras such as the Panaflex, Arriflex and Eclair reflex cameras. In reflex motion picture cameras the cameraman views the scene through the same optical system that projects the image onto the motion picture film, rather than through a separate viewfinder incorporating a completely separate lens system. The cameraman may thus see the scene the film sees and adjust focus and convergence as required. Such cameras present tighter and more exacting packaging problems compared to non-reflex motion picture cameras owing to the space occupied by the reflex mirror and often to smaller camera flange diameters. Thus, both the displacement prisms of the Bernier system and the lenses provided therewith must fit into a smaller space than is the case with the viewfinder Mitchell camera and the like. The reflex mirror requires increased back focal lengths for lenses in reflex cameras. Another factor requiring minimization of the size of the prisms and lenses of the Bernier type optical attachment for use with modern reflex cameras is the weight of the overall system, which should be decreased for greater portability and convenience of use.

The lens system used with non-reflex motion cameras such as the Mitchell BNC is typified by the trioptiscope lens system which has four elements in three groups (the rearmost group consists of two elements cemented together). This lens system is incapable of fitting into the three dimensional photography attachment necessary for reflex motion picture cameras due to mechanical interference with the reflex mirror as well as other portions of the camera. In addition, the lenses provided for use with the Bernier attachment for non-reflex motion picture cameras have not been available in a variety of focal lengths. Three dimensional cinematography would be materially enhanced by the availability of wide angle and telephoto lens systems in addition to normal lens systems. Such lenses should be at least of the quality of the trioptiscope lens provided hitherto in order to be adequate for motion picture photography.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide lens systems adapted for use in three dimensional Bernier type photography attachments for reflex cameras.

Another object of the present invention is to provide lens systems of diverse effective focal lengths for use with reflex motion picture cameras in three dimensional cinematography.

A particular object of the present invention is to provide wide angle, telephoto, and normal lens systems for use with reflex motion picture cameras in three dimensional cinematography.

Still another object of the present invention is to provide lens systems for use with reflex motion picture cameras in three dimensional cinematography which are of high optical quality as well as being small in size and low in weight. Other and further objects and advantages will appear hereinafter.

According to the present invention, lens systems are provided which comprise a sequence of a negative lens followed by a triplet lens (a triplet lens is a compound lens composed of three single lens elements). A sequence of four powered single lenses having the following power sequence is preferred: negative, positive, negative and positive. Two lens systems, mounted parallel and adjacent to each other, comprise a version of the invention adapted for use in three-dimensional photography attachments to reflex cameras.

The invention provides particular lens systems of various effective focal lengths. In particular, a normal lens system, a wide angle lens system which includes plane parallel refractive elements between the first negative lens and the first positive lens and between the first positive lens and the second negative lens, and a telephoto lens are provided. All of these lens systems allow for the greatest possible distance between the image plane and the first element (through which light passes) of the lens system to minimize the size of the front displacement prisms while also satisfying the large back focal length requirement. In the telephoto version of the invention, internal displacement prisms are provided which transmit images from the telephoto lens system to the image plane through the camera flange entrance.

The invention also may be adapted for use with three-dimensional reflex television cameras. In such cameras the right and left hand images are recorded electronically, rather than on motion picture film. Although this disclosure primarily describes the invention in the context of three-dimensional cinematography, for use with standard reflex motion picture cameras, the invention is not limited to such uses and includes television applications.

The most important features of the invention have been outlined very broadly. Additional features of the invention exist that will be set forth hereinafter in the detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the invention have been chosen for purposes of illustration and are shown in the accompanying drawings wherein:

FIG. 1 is a longitudinal section of the trioptiscope lens system of the prior art, with illustrative ray diagrams showing the refraction of light thereby, and FIG. 2 comprises the modulation transfer function curves for this lens system;

FIGS. 3, 5, and 7 are longitudinal sections of the normal lens system, wide angle lens system, and telephoto lens system preferred embodiments of the invention, respectively, with illustrative ray diagrams showing the refraction of light thereby, and FIGS. 4, 6, and 8 are the corresponding modulation transfer function graphs;

FIG. 9 is a longitudinal section of two telephoto lens systems mounted in parallel allied with internal displacement prisms, in the general arrangement required by the Bernier system; and

FIG. 10 is an oblique perspective view of an internal displacement prism used with the telephoto lens system of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The trioptiscope lens system, an example of the prior art, is shown in FIG. 1. Its modulation transfer function curves are shown in FIG. 2. All data presented for the performance of the lens system of FIG. 1 and the lens systems of the present invention include image data on an axis field point, 0.7 of the format corner field point, and the format corner field point. Appendix I contains a table presenting the dimensions and characteristics of the trioptiscope lens focused at infinity, for a 35 mm movie film format.

From the above data and FIG. 1 one may observe that the trioptiscope lens system is composed of four lens elements in three groups, of which the last lens group is a doublet. The three groups have the following power sequence: positive, negative, and positive. The first lens element is convex-planar, the second biconcave and the doublet is a combination of, first, a bioconcave, and second, a biconvex lens.

The trioptiscope lens system is compatible with use in viewfinder cameras such as the Mitchell BNC. Modern reflex motion picture cameras, such as the Panavision Panaflex camera, present more stringent interface requirements for lens systems used in Bernier-type three-dimensional cinematography attachments.

The lens systems of the present invention will meet the interface requirements of modern reflex motion picture cameras, used in a three-dimensional cinematography attachment containing two such lens systems adjacent and parallel to each other.

Referring to FIGS. 3, 5, and 7, the lens systems of the preferred embodiments of the invention comprise four single lenses L₁ -L₄ having spherical surfaces, sequentially arranged in the direction of travel of light through the lens system from object to image plane I. FIG. 3 shows a normal lens system embodiment of the invention, FIG. 5 a wide angle lens system embodiment, and FIG. 7 a telephoto lens system embodiment. The first lens L₁ is a meniscus lens of negative focal length with its convex surface facing the object. The second lens L₂ is a biconvex lens for the normal and wide angle lens systems or a meniscus lens of positive focal length for the telephoto lens system, with at least one surface convex to the object side of the lens system. The third lens L₃ is a bioconcave lens of negative focal length. The fourth lens L₄ is a biconvex lens for the telephoto lens system, or, for the normal and wide angle lens systems, a meniscus lens of positive focal length with its surfaces convex to the film or image plane I.

Two such lens systems are mounted in parallel and adjacent to each other to form the overall lens system needed for three dimensional cinematography in a reflex motion picture camera. The particular dimensions, indices of refraction, separations, and alignment of the single lens elements produce normal, wide angle or telephoto lens systems. (The telephoto lens system of the present invention is actually an inverted telephoto lens system but will be referred to as a telephoto system for convenience.) Plane parallel refractive optical elements E₁ and E₂ are provided in the wide angle lens system of the present invention, FIG. 5, to maximize the distance from the forward (forward referring to the location closest to the object) lens element to the image plane I while minimizing the lens element diameters. In addition, because of the large overall width of the telephoto lens system and its mounting, internal displacement prisms, shown in FIGS. 9 and 10, are used for bringing the right and left eye images within the camera flange opening for superposition at the correct lateral separation upon the film. All air-glass surfaces preferably are provided with magnesium fluoride or other anti-reflection coatings.

In FIGS. 3, 5, and 7 the letters d₁, d₂, . . . d_(i) indicate axial separations between successive refracting surfaces. The same symbology is used with respect to lens elements in FIG. 1, which shows the trioptiscope lens system.

The same or substantially similar optical results for the three dimensional photography attachment preferred embodiments presented herein may be obtained with variations of the four powered element designs used for the lens systems. The four powered element lens system design is the simplest configuration, with the fewest number of powered elements. Other lens configurations, with the same basic optical theme, could be used to obtain the basic results of this invention.

The inverted telephoto designs for the normal, wide, and telephoto lens preferred embodiments of the invention presented herein may be described as a triplet lens (elements two through four) preceded by a negative lens (first element). Variations of this theme would be inverted telephoto lenses that employ hybrid triplet lenses after the first negative lens; or compound (split) negative elements before the triplet lens; or both:

(A) The simplest variation would be a negative element followed by a triplet hybrid known as a Tessar (the Tessar triplet is actually a four element lens systems in which the two rear elements are cemented together to form a dual component combinations len). This would be a five element lens system having the following power arrangement: negative, positive, negative, negative, and positive.

(B) Another arrangement would be a negative element followed by the triplet hybrid known as a Pentac-Heliar (the Pentac-Heliar triplet is actually a five element lens system in which the two front elements and the two rear elements are cemented together, respectively, to form two dual component combination lenses flanking a single negative power lens element). This would be a six element lens system having the following power arrangement: negative, positive, negative, negative, negative, and positive.

(C) Another simple variation would be splitting the negative element into two weak negative elements (either cemented together or not) followed by the triplet configuration. This would be a five element lens system having the following power arrangement: negative, negative, positive, negative, and positive.

Other variations would be combinations of situations A, B, and C as described above; or with the various doublet groups uncemented.

All of these basic variations have the potential of achieving imagery the same as or better than the four element lens system designs, while also fitting the flange requirements of today's reflex motion picture cameras with nearly the same or more total weight of the four element lens system design preferred embodiments presented herein. Reasons to design the lens systems according to the variations would be to obtain even better image quality; or to make them faster; or to avoid the high index glasses used in the four element lens systems of the preferred embodiments presented herein. Although the lens systems of the preferred embodiments are combinations of four single lens powered elements, the invention is understood to not be limited to the use of single lens powered elements, but may include the use of equivalent compound or multi-component lens powered elements in place of single lens powered elements.

FIG. 3 depicts the normal lens system preferred embodiment of the present invention and FIG. 4 shows its modulation transfer function curves. The higher modulation transfer function of the normal lens system of the present invention, compared to the trioptiscope lens system, means that better image quality will result.

Parameters of the normal lens system preferred embodiment (FIG. 3) of the present invention, compatible with a 35 mm movie film format, are given in the following table:

    ______________________________________                                         F/4.5 Normal lens Focused at Infinity                                          Element                                                                               R.sub.1  R.sub.2  d      CA.sub.1                                                                            CA.sub.2                                                                            GLASS                                ______________________________________                                         1      15.700   7.269     .750  7.800                                                                               7.044                                                                               LASF                                                          1.443                                                 2      9.357    -35.989  2.650  7.583                                                                               7.540                                                                               SF7                                                           1.880                                                           APERTURE STOP                                                                               7.167                                                                            1.086                                                 3      -11.109  37.501   1.250  7.076                                                                               7.501                                                                               SF57                                                           .542                                                 4      -69.872  -9.044   2.650  7.647                                                                               8.500                                                                               LASF5                                                         36.066                                                IMAGE                FLAT     23.941                                           ______________________________________                                         EFL                 =     31.600                                               BFL                 =     36.066                                               FFL                 =     -19.084                                              F/No                =     4.500                                                OAL                 =     12.254                                               SEMI-FIELD ANGLE    =     20.748°                                       ENTRANCE PUPIL                                                                 DIAMETER            =     7.022                                                DISTANCE            =     5.031                                                EXIT PUPIL                                                                     DIAMETER            =     9.201                                                DISTANCE            =     -5.341                                               ______________________________________                                          (Dimensions are given in millimeters. Definitions of the symbols used for      this and subsequent lens prescription tables will be found in Appendix I.                                                                               

The normal lens system embodiment of the present invention has an f-number of 4.5 and an effective focal length of 32 millimeters in the 35 mm movie film format. Mechanical variation of the stop diameter, provided at the aperture stop, will allow variation of f-number from no faster than f/4.5 down to whatever slower f-number is desired and mechanically designed into the lens. In the table presented in Appendix II, the critical dimensions and other characteristics of the normal lens system of the present invention are compared to the trioptiscope lens system of the prior art, which is also an f/4.5, 32 millimeter effective focal length lens system. "Prism" refers to the Bernier displacement prism or "external displacement prism" as it will be called hereinafter; interocular separation, also known as stereo base, refers to the ocular separation of the image forming light received by the two entrance faces of the external displacement prisms, which is responsible for the ultimate three-dimensional effect.

The lens systems of the present invention may also be scaled for use in other format reflex cameras such as the 65 mm movie film format.

Scaling up affects the pulldown (the amount of film drawn down in front of the lens systems for a single exposure) and the intraocular distance.

The normal framing aspect ratio of width to height of the picture frame for a typical 35 mm reflex motion picture camera, used with lens systems of the present invention, is 2.36:1 for both channels. The 35 mm camera aperture dimensions are 0.735×0.868 inch for the height and width, respectively. Although the 35 mm format lens system design of the present invention may be easily scaled for use with 65 mm motion picture cameras, resolvable difficulties arise in achieving the same effective wide screen image. The most common 65 mm cameras use five perforation pulldown (perforations referring to the apertures in the sides of the film strip). However, a ten perforation pulldown camera needs to be used in order to achieve the same projected effective wide screen image for theater viewing while exploiting the available film width. The 65 mm ten perforation pulldown camera would use a 1.81×2.07 inch height and width camera aperture on which to record both channels. This would yield a framing aspect ratio similar to that of the 35 mm system, namely 2.29:1. The height could then be matted down to use 97% of the camera aperture area to exactly match the 35 mm framing aspect ratio of 2.36:1.

In scaling the lens systems of the present invention to the 65 mm three-dimensional motion picture application, the interocular distance also scales. If left at the scaled value, this situation would yield an enhanced stereoscopic effect upon viewing a picture in a theater when compared with the scene taken with the 35 mm camera lens covering the same field of view. This need not be the case since the interocular distance could be designed (via external displacement prism design modification) to be the same as for the 35 mm camera, or nearly so, depending on the selection of the appropriate ten perforation camera and/or modified ten perforation camera. This would also mean that the external displacement prisms for 65 mm camera use would not be constructed according to the scaled-up design of a scaled 35 mm camera external displacement prism, although the concept would be identical. Redesign of the external displacement prisms will not alter the field of view of the camera; narrowing the interocular separation through redesign of the external displacement prisms will also not narrow the field of view.

An example of a lens prescription that could be used for 65 mm filming is given in the table below, in which length dimensions are given in millimeters. This lens would have a 75.4 mm focal length and cover a total field of 41.5 degrees (the same field of view as the 35 mm camera case). The purely scaled Bernier external displacement prisms would have all the linear dimensions of the 35 mm camera version scaled by times 2.39; the angular dimensions would be the same. The scaled interocular distance would be 155 mm but could be made to be the 35 mm camera value of 65 mm depending on modification of the filming camera to accept the redesigned prisms for the 65 mm camera format.

    ______________________________________                                         f/4.5 Normal Lens Infinite Focus; 65 mm Format                                 Ele-                                                                           ment  R.sub.1   R.sub.2  d     CA.sub.1                                                                             CA.sub.2                                                                             GLASS                               ______________________________________                                         1     37.478    17.353   1.790 18.711                                                                               16.887                                                                               LASF5                                                        3.446                                                 2     22.336    -85.906  6.325 18.381                                                                               18.065                                                                               SF7                                                          4.489                                                           APERTURE   17.107                                                              STOP                                                                                          2.594                                                 3     -26.517   89.515   2.983 16.886                                                                               17.893                                                                               SF57                                                         1.295                                                 4     -166.785  -21.589  5.325 18.270                                                                               20.338                                                                               LASF5                                                        85.728                                                IM-                  FLAT    58.650                                            AGE                                                                            ______________________________________                                         EFL                 =     75.429                                               BFL                 =     86.090                                               FFL                 =     -45.554                                              F/N                 =     4.500                                                IMAGE DIST          =     85.728                                               OAL                 =     29.250                                               SEMI-FIELD ANGLE    =     21.244°                                       ENTRANCE PUPIL                                                                 DIAMETER            =     16.762                                               DISTANCE            =     12.009                                               EXIT PUPIL                                                                     DIAMETER            =     21.964                                               DISTANCE            =     -12.749                                              ______________________________________                                    

The normal lens system preferred embodiments of the present invention, discussed above, may be described in terms of dimensionless lens prescriptions by dividing all lengths by the effective focal length and multiplying by 100. The table below gives the dimensionless lens prescriptions obtained thereby.

    ______________________________________                                         Effective focal length = 100.0, f/4.5,                                         semi-field angle = 20.7°                                                back focal length = 114.1                                                      aperture stop located 5.95 behind second lens                                  Lens  R.sub.i     d.sub.i    n.sub.i ν.sub.i                                ______________________________________                                         L.sub.1                                                                              R.sub.1 = 49.7                                                                             d.sub.1 = 2.37                                                                            n.sub.1 = 1.8807                                                                       ν.sub.1 = 41.01                              R.sub.2 = 23.0                                                                             d.sub.2 = 4.57                                                                            Airspace                                          L.sub.2                                                                              R.sub.3 = 29.6                                                                             d.sub.3 = 8.39                                                                            n.sub.2 = 1.6398                                                                       ν.sub.2 = 34.61                              R.sub.4 = -113.9                                                                           d.sub.4 = 22.42                                                                           Airspace                                          L.sub.3                                                                              R.sub.5 = -35.2                                                                            d.sub.5 = 3.96                                                                            n.sub.3 = 1.8467                                                                       ν.sub.3 = 23.83                              R.sub.6 = 118.7                                                                            d.sub.6 = 1.72                                                                            Airspace                                          L.sub.4                                                                              R.sub.7 = -221.1                                                                           d.sub.7 = 8.39                                                                            n.sub.4 = 1.8807                                                                       ν.sub.4 = 41.01                              R.sub.8 = -28.6                                                          ______________________________________                                    

(Where L₁, L₂, L₃, and L₄ represent the first to fourth single lenses, respectively, the distance the aperture stop is located behind the second lens L₂ is the axial separation between the aperture stop and the surface of L₂ nearest the image side of the lens system, R₁, R₂, . . . R₈ represent the radii of curvature of the respective lens surfaces; d₁, d₂, . . . d₇ represent the axial separations between consecutiive refracting surfaces; n₁, n₂, . . . n₄ represent the refractive indexes of each lens for D-line of sodium; and ν₁, ν₂, . . . ν₄ are the Abbe numbers of the respective single lenses.)

The lens system of the present invention is capable of being adapted to have different focal lengths, yet still be accommodated successfully in modern reflex motion picture cameras having different film formats.

The wide angle lens system preferred embodiment of the invention is depicted in FIG. 5 and its modulation transfer function curves are shown in FIG. 6. The plane parallel refractive elements E₁ and E₂ are formed of high index, low dispersion glass in order to physically lenghthen the lens system yet retain excellent image quality. Lengthening the lens is essential to maximizing the entrance pupil to film distance despite the short focal length.

Parameters of the wide angle lens system preferred embodiment of the present invention, for a 35 mm movie film format, are given in the following table:

    ______________________________________                                         F/4.5 Wide Angle Focused at Infinity                                           Element                                                                               R.sub.1  R.sub.2  d      CA.sub.1                                                                            CA.sub.2                                                                            GLASS                                ______________________________________                                         1        15.463   7.073  .7500  8.418                                                                               7.600                                                                               LASF5                                                         1.4259                                                2      IN-      IN-      5.4972 7.555                                                                               7.147                                                                               LASF5                                       FINITY   FINITY   .2500                                                 3        14.612 -57.679  1.2802 7.051                                                                               6.969                                                                               LASF9                                                         .3750                                                           APERTURE STOP                                                                              6.905                                                                             .3750                                                 4      IN-      IN-      5.6148 6.854                                                                               6.678                                                                               LASF5                                       FINITY   FINITY   .5329                                                 5       -25.717  20.667  1.0000 6.856                                                                               7.548                                                                               SF57                                                          .6749                                                 6      -397.296  -9.758  1.7281 7.985                                                                               8.520                                                                               LAK17                                                         29.9800                                               IMAGE                FLAT     23.9418                                          ______________________________________                                         EFL                 =     24.000                                               BFL                 =     29.980                                               FFL                 =     -10.207                                              F/No                =     4.500                                                OAL                 =     19.503                                               SEMI-FIELD ANGLE    =     26.509                                               ENTRANCE PUPIL                                                                 DIAMETER            =     5.333                                                DISTANCE            =     4.761                                                EXIT PUPIL                                                                     DIAMETER            =     8.551                                                DISTANCE            =     -8.500                                               ______________________________________                                    

A dimensionless lens system prescription for the above wide angle lens system preferred embodiment of the present invention (in terms of multiples of the effective focal length multiplied in turn by 100) is given below.

    ______________________________________                                         Effective focal length = 100.0 f/4.5                                           back focal length = 124.9                                                      semi-field angle = 26.5°                                                aperture stop located 1.56 behind second lens                                  Element                                                                               R.sub.i     d.sub.i   n.sub.i ν.sub.i                                ______________________________________                                         L.sub.1                                                                               R.sub.1 = 64.4                                                                             d.sub.1 = 3.12                                                                           n.sub.1 = 1.8807                                                                       ν.sub.1 = 41.01                               R.sub.2 = 29.5                                                                             d.sub.2 = 5.94                                                                           Airspace                                          E.sub.1                                                                               R.sub.3 = infinity                                                                         d.sub.3 = 22.91                                                                          n.sub.2 = 1.8807                                                                       ν.sub.2 = 41.01                               R.sub.4 = infinity                                                                         d.sub.4 = 1.04                                                                           Airspace                                          L.sub.2                                                                               R.sub.5 = 60.9                                                                             d.sub.5 = 5.33                                                                           n.sub.3 = 1.8503                                                                       ν.sub.3 = 32.23                               R.sub.6 = -240.3                                                                           d.sub.6 = 3.13                                                                           Airspace                                          E.sub.2                                                                               R.sub.7 = infinity                                                                         d.sub.7 = 23.40                                                                          n.sub.4 = 1.8807                                                                       ν.sub.3 =  41.01                              R.sub.8 = infinity                                                                         d.sub.8 = 2.22                                                                           Airspace                                          L.sub.3                                                                               R.sub.9 = -107.2                                                                           d.sub.9 = 4.17                                                                           n.sub.5 = 1.8467                                                                       ν.sub.5 = 23.83                               R.sub.10 = 86.1                                                                            d.sub.10 = 2.81                                                                          Airspace                                          L.sub.4                                                                               R.sub.11 = -1655.4                                                                         d.sub.11 = 7.20                                                                          n.sub.6 = 1.7885                                                                       ν.sub.6 = 50.48                               R.sub.12 = -40.7                                                        ______________________________________                                    

(Where L₁, L₂, L₃, and L₄ represent the first to fourth single lenses, respectively, E₁ and E₂ represent the plane parallel refractive elements, the distance the aperture stop is located behind the second lens L₂ is the axial separation between the aperture stop and the surface of L₂ nearest the image side of the lens system, R₁, R₂, . . . R₁₂ represent the radii of curvature of the respective lens and element surfaces, d₁, d₂, . . . d₁₁ represent the axial separations between consecutive lens and element surfaces, n₁, n₂, . . . n₆ represent the refractive indexes of each lens or element for D-line of sodium, and ν₁, ν₂, . . . ν₆ are the Abbe numbers of the respective lenses or elements.)

The telephoto lens system preferred embodiment of the invention is depicted in cross-section in FIG. 7 and its modulation transfer function curves are shown in FIG. 8. The telephoto lens system is more precisely called an inverted telephoto lens system; it is used for long views and to create a narrow field of view. In FIG. 7 the internal replacement prism is illustrated by the slab of glass indicated as IDP. This slab represents the unfolded internal displacement prism pertinent to this lens system. The lens system is designed to compensate for the effects of having this amount of glass following the lens system. An internal displacement prism is represented, as it actually appears, in FIG. 10.

Parameters of the telephoto lens system preferred embodiment, plus an associated internal displacement prism or IDP, for the 35 mm movie film format, are given in the following table. (The internal displacement prism will be discussed in greater detail later.)

    ______________________________________                                         F/4.5 Telephoto Focused At Infinity                                            Ele-                                                                           ment  R.sub.1  R.sub.2  d     CA.sub.1                                                                             CA.sub.2                                                                             GLASS                                ______________________________________                                         1      44.014   12.770  1.000 12.601                                                                               11.750                                                                               FK5                                                          2.385                                                  2      14.331   162.200 2.068 11.808                                                                               11.640                                                                               LAFN2                                                        4.247                                                           APERTURE   10.507                                                              STOP                                                                                          3.497                                                  3     -22.416   20.067  1.000  9.696                                                                                9.778                                                                               SF4                                                          1.426                                                  4      35.940  -17.878  2.070 10.360                                                                               10.556                                                                               LAFN2                                                        45.983                                                                         0.500                                                  IDP   IN-      IN-      25.600                                                                               10.900                                                                               14.875                                                                               SF10                                       FINITY   FINITY   30.570                                                 IM-                 FLAT    23.9418                                            AGE                                                                            ______________________________________                                         EFL                 =     50.000                                               BFL                 =     30.570                                               FFL                 =     -37.218                                              F/No                =     4.500                                                OAL                 =     17.694                                               SEMI-FIELD ANGLE    =     13.464°                                       ENTRANCE PUPIL                                                                 DIAMETER            =     11.111                                               DISTANCE            =     8.540                                                EXIT PUPIL                                                                     DIAMETER            =     12.140                                               DISTANCE            =     -8.650                                               ______________________________________                                    

A dimensionless lens system prescription for the above telephoto lens system embodiment of the present invention and an associated internal displacement prism (in terms of multiples of the effective focal length, f, multiplied in turn by 100) is given below.

    ______________________________________                                         Effective focal length = 100.0                                                 F/4.5 semi-field angle = 13.5°                                          back focal length = 61.1                                                       aperture stop located 8.49 behind second lens                                  Lens R.sub.i     d.sub.i  nhd i     ν.sub.i                                 ______________________________________                                         L.sub.1                                                                             R.sub.1 = 88.0                                                                             d.sub.1 = 2.0                                                                           n.sub.1 = 1.4875                                                                         ν.sub.1 = 70.41                              R.sub.2 = 25.5                                                                             d.sub.2 = 4.8                                                                           Airspace                                             L.sub.2                                                                             R.sub.3 = 28.7                                                                             d.sub.3 = 4.1                                                                           n.sub.2 = 1.7440                                                                         ν.sub.2 = 44.77                              R.sub.4 = 324.4                                                                            d.sub.4 = 15.5                                                                          Airspace                                             L.sub.3                                                                             R.sub.5 = -44.8                                                                            d.sub.5 = 2.0                                                                           n.sub.3 = 1.7552                                                                         ν.sub.3 = 27.58                              R.sub.6 = 40.1                                                                             d.sub.6 = 2.9                                                                           Airspace                                             L.sub.4                                                                             R.sub.7 = 71.9                                                                             d.sub.7 = 4.1                                                                           n.sub.4 = 1.7440                                                                         ν.sub.4  = 44.77                             R.sub.8 = -35.8                                                                            d.sub.8 = 1.0                                                                           Airspace                                             IDP  R.sub.9 =   d.sub.9 = 51.2                                                                          n.sub.IDP = 1.7282                                                                       ν.sub.IDP = 28.41                            INFINITY                                                                       R.sub.10 =                                                                     INFINITY                                                                  ______________________________________                                    

(Where L₁, L₂, L₃, and L₄ represents the first to fourth single lenses, IDP represents the internal displacement prism, the distance the aperture stop is located behind the second lens L₂ is the axial separation between the aperture stop and the surface of L₂ nearest the image side of the lens system, R₁, R₂, . . . R₈ represent the radii of curvature of the respective lens surfaces, R₉ and R₁₀ represent the radii of curvature of the IDP entrance and exit faces, respectively, d₁, d₂, . . . d₉ represent the axial separations between consecutive refracting surfaces, n₁, n₂, . . . n₄ represent the refractive indexes of each single lens for D-line of sodium, n_(IDP) represents the refractive index of the IDP, ν₁, ν₂, . . . ν₄ are the Abbe numbers of the respective single lenses, and ν_(IDP) is the Abbe number of the IDP.)

The telephoto lens system preferred embodiment of the present invention has an entrance pupil larger than may be accommodated within the reflex camera flange opening when the lens system is placed as it usually is in the Bernier mechanism; therefore it is used with internal displacement prisms in the arrangement shown in FIG. 9, which is a cross-section along a vertical plane (which is the plane at right angles to the image plane). FIG. 9 shows how internal displacement prisms P₁ and P₂ gather light from the two telephoto lens systems T₁ and T₂ and pass it backwardly and inwardly within camera flange opening O. This is illustrated by ray diagrams in FIG. 9 which culminate at image plane I within the camera. B indicates the mount within which the three-dimensional cinematography attachment is contained, which generally resembles an open cone with ledges inside as seen by eye from the front of the camera, but in the cross-section of FIG. 9 resembles two stepped sides of a triangle. The term "backwardly" refers to the general direction from lens systems to film plane within the camera; "inwardly" means towards an axis parallel to and directly between the axes of the two lens systems.

The external displacement prisms of the Bernier three-dimensional cinematography arrangement are not shown in FIG. 9 but would be to the left in FIG. 9 (reference should be made to FIGS. 2 and 10 of Bernier, U.S. Pat. No. 3,531,191). The parallel and adjacent alignment of the two telephoto lens systems is characteristic of the Bernier arrangement; the wide angle and normal lens systems of the present invention are disposed likewise when used in three-dimensional cinematography.

FIG. 10 depicts a single internal displacement prism for use with the telephoto lens system preferred embodiment of the present invention. The internal displacement prism has an entrance face 1 and an exit face 4 with heights 2, 5 and widths 3, 6 respectively. The entrance and exit faces are flat. The internal displacement prism has first and second reflection faces 7 and 8 which are oriented at an angle θ with respect to the planes of the entrance and exit faces 1 and 4 (which are themselves parallel). Reflection faces 7 and 8 cause light rays entering entrance face 1 to be displaced axially (or inwardly, as is best seen in FIG. 9) upon leaving exit face 4. The particular internal displacement prism shown in FIG. 10 is a compound prism, formed by joining two simpler prisms at a face 10 by cementing. Also, prism surfaces other than those used by the intended light path are fine ground plus have opaque coatings to help suppress stray light.

The internal displacement prisms will be in contact with each other at faces 11 (see FIG. 9) when placed within the camera flange opening. This will necessitate a small amount (<5%) of additional vignetting on the film, on the sides of the frames common to both channels (i.e., common to both right and left hand images).

The following table gives an example of the dimensions of such an internal displacement prism adapted for use with the telephoto lens system of the previous example, for a 35 mm movie film format.

    ______________________________________                                         Internal Displacement Prism                                                    Parameter          Value                                                       ______________________________________                                         Entrance Face, H × W                                                                        10.5 mm × 14.0 mm                                     Exit face, H × W                                                                            9.56 mm × 15.5 mm                                     Total Axial Displacement                                                                          10.15 mm                                                    Maximum Lower Half Exit                                                        Face Aperture Height                                                                              4.6 mm                                                      Reflection Face Orientation (θ)                                                             50.0°                                                Glass              728284                                                      ______________________________________                                    

The normal lens system, wide angle lens system, and telephoto lens system preferred embodiments of the present invention, for 35 mm movie film format, are summarized and compared in the table of Appendix III.

The external displacement prisms are smaller and lighter than those used with lens systems of the prior art, but vary in dimension and interocular separation according to the focal length of the lens system. The external displacement prisms for the normal and wide angle lenses are made of 728284 glass and the telephoto prisms are made of 620364 glass.

The wide angle and telephoto lens systems preferred embodiments of the present invention may be easily scaled up for use in a 65 mm film format, as was discussed previously for the normal lens system of the present invention. A pure scaling up of the wide angle and telephoto external displacement prism designs intended for 35 mm format cameras would result in 65 mm versions having interocular separations of 119 mm and 243 mm for the wide angle and telephoto lens systems, respectively. The same considerations regarding effective wide screen angle, framing aspect ratio, and pulldown would apply as for the normal lens system scale-up.

The wide angle and telephoto lens systems of the present invention further embody unusual geometry in order to achieve the tight packaging goals necessary. These goals are accomplished by incorporation of the following concepts:

A. For the wide angle:

1. An approximately f/4.5 objective lens system;

2. With back focal length greater than or equal to 1.16 effective focal length;

3. Covering a 26° or greater semi-field;

4. With four or more optical elements;

5. With a maximum clear aperture diameter no larger than 60% greater than the entrance pupil diameter;

6. With corner illumination greater than 40%; and

7. Having a sum of axial glass thicknesses over 80% of the total length from front vertex to back vertex.

B. For the telephoto lens:

1. An approximately f/4.5 objective lens system;

2. With back focal length 80% of the effective focal length;

3. Covering a 13.5° or greater semi-field;

4. Having four or more optical elements;

5. With maximum clear aperture no larger than 115% of the entrance pupil diameter;

6. Having corner illumination greater than 40%; and

7. With an overall length from front vertex to back of less than 38% of the effective focal length.

Thus, lens systems have been provided that provide high quality optical images, in conjunction with their use in three dimensional photography attachments to reflex motion picture cameras. Those skilled in the art will appreciate that the conception upon which this disclosure is based may be used as a basis for the designing of other structures, for carrying out the several purposes of the invention. The claims, therefore, should be regarded as including such equivalent constructions as do not depart from the spirit and scope of the invention, which is intended to be defined by the appended claims. 

What is claimed is:
 1. A lens system comprising first to fourth single lenses, said first and third single lens being of negative focal length, said first single lens being a meniscus lens convex to the object side of the lens system, and said third single lens being a biconcave lens, said second and fourth single lens being of positive focal length, said second single lens being biconvex and said fourth single lens being a meniscus lens concave to the object side of the system, in which the lens system satisfies the following data:

    ______________________________________                                         Effective focal length = 100.0 f/4.5                                           semi-field angle = 20.75°                                               back focal length = 114.1                                                      Lens  R.sub.i     d.sub.i    n.sub.i ν.sub.i                                ______________________________________                                         L.sub.1                                                                              R.sub.1 = 49.7                                                                             d.sub.1 = 2.37                                                                            n.sub.1 = 1.8807                                                                       ν.sub.1 = 41.01                              R.sub.2 = 23.0                                                                             d.sub.2 = 4.57                                                                            Airspace                                          L.sub.2                                                                              R.sub.3 = 29.6                                                                             d.sub.3 = 8.39                                                                            n.sub.2 = 1.6398                                                                       ν.sub.2 = 34.61                              R.sub.4 = -113.9                                                                           d.sub.4 = 22.42                                                                           Airspace                                          L.sub.3                                                                              R.sub.5 = -35.2                                                                            d.sub.5 = 3.96                                                                            n.sub.3 = 1.8467                                                                       ν.sub.3 = 23.83                              R.sub.6 = 118.7                                                                            d.sub.6 = 1.72                                                                            Airspace                                          L.sub.4                                                                              R.sub.7 = -221.1                                                                           d.sub.7 = 8.39                                                                            n.sub.4 = 1.8807                                                                       ν.sub.4 = 41.01                              R.sub.8  = -28.6                                                         ______________________________________                                    

where L₁, L₂, L₃, and L₄ represent the first to fourth single lenses, respectively, R₁, R₂, . . . R₈ represent the radii of curvature of the respective lens surfaces; d₁, d₂, . . . d₇ represent the axial separations between consecutive refracting surfaces; n₁, n₂, . . . n₄ represent the refractive indexes of each lens for D-line of sodium; and ν₁, ν₂, . . . ν₄ are the Abbe numbers of the respective single lenses.
 2. The lens system of claim 1 further comprising an aperture stop disposed between the second and third single lenses.
 3. The lens system of claim 2 in which the aperture stop has an axial separation of 5.95 from the second lens L₂. 